Gordian knot amplitude antinode block body cake clump cluster concrete concretion conglomerate conglomeration crest crux de Broglie wave diffraction dilemma electromagnetic radiation electromagnetic wave enigma frequency frequency band frequency spectrum guided wave in phase interference knot knotty point light longitudinal wave lump mass mechanical wave nodosity nodulation nodule nodulus nodus nonplus out of phase oxymoron paradox period periodic wave perplexity pons asinorum poser puzzle quandary radio wave ray reinforcement resonance resonance frequency seismic wave shock wave solid solid body sound wave surface wave teaser tidal wave transverse wave trough vexed question wave wave equation wave motion wave number wavelength